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NASA's Deep Impact spacecraft may have flown within 435 miles
(700 km) of Comet Hartley 2 today (Nov. 4), but it wasn't the
first time a robotic emissary from Earth cozied up to an icy
wanderer. In fact, Deep Impact is the fifth probe to image a
comet up-close, with a sixth mission yet to come.

Astronomers hope
Deep Impact's rendezvous which took place at 10:01 a.m. EDT
(1401 GMT) today (Nov. 4) will teach them more about comet
structure and behavior, and perhaps yield clues about the early
days of the solar system.

Researchers are eager to compare the Hartley 2 data with
information gleaned from the previous flybys. And they'll
doubtless learn much more when the European Space Agency's
Rosetta probe
drops a lander on Comet 67P/Churyumov-Gerasimenko in 2014.

The first spacecraft to visit a comet was the International
Cometary Explorer, which zipped through the tail of Comet
Giacobini-Zinner in September 1985. ICE was originally launched
in 1978, as part of the International Sun-Earth Explorer mission
to study Earth's magnetosphere and its interaction with the solar
wind.

ICE then turned its instruments on Comet Halley in 1986,
observing the ice ball from a distance of 17.4 million miles (28
million km).

After ICE and before Deep Impact's latest encounter, four
missions managed to take up-close photos of comet nuclei. Here's
a brief rundown of those four, along with Deep Impact's mission:

The famed Halley's Comet was the first comet a spacecraft imaged
up close. In 1986, the ESA's Giotto probe zoomed to within about
372 miles (600 km) of the icy wanderer's nucleus. Four other
spacecraft also visited Halley that year two each from the Soviet
Union and Japan but none approached as close as Giotto, according
to NASA.

Giotto returned a lot of useful information, finding that the
comet's nucleus is rough, porous, dark and dusty. The probe's
data also helped scientists determine that Halley is made of some
of the oldest stuff in the solar system, volatiles that condensed
onto dust particles about 4.5 billion years ago.

Halley is about 9 miles (15 km) long by 5 miles (8 km) wide or
so. It completes a circuit around the sun every 75 or 76 years.
It should return to the inner solar system around 2051.

Comet Borrelly

NASA's Deep Space 1 probe flew to within 1,364 miles (2,200
km) of Comet Borrelly in September 2001. The spacecraft returned
dazzling and surprising photos, showing rolling, pitted terrain
marked by grand mesas.

Deep Space 1's pictures of the potato-shaped Borrelly were hailed
by scientists as the best yet taken of a comet. These images
showed that Borrelly is even darker than Halley, reflecting just
half as much light as the surface of the moon.

Comet Borrelly is about 5 miles (8 km) long and makes a complete
trip around the sun once every 6.9 years.

Stardust observed lots of cliffs and hills on the comet's
surface, as well as active, gas-spewing vents. The probe also
collected some dust from Wild 2's coma the cloud surrounding the
comet's nucleus and brought the stuff back to Earth.

Wild 2 is a small comet, measuring just 3.1 miles (5 km) across.
It makes a full lap around the sun every 6.4 years or so.

The Deep Impact spacecraft served as mothership for NASA's
mission to Comet Tempel 1, which crashed an 820-pound
(371-kilogram) probe into the ice ball in 2005. [ Best
Deep Impact Comet Crash Photos ]

The impact revealed a great deal of water inside and on the
surface of Tempel 1, as well as many organic molecules the
building blocks of life in its interior. Researchers also got
glimpses of layered, primordial material within the comet,
yielding clues to how the comet may have formed 4.5 billion years
ago.

Tempel 1 is about 4.3 miles (7 km) across and has an orbital
period of 5.5 years.

Comet Hartley 2

Deep Impact has been chasing Hartley 2 for months. Today (Nov.
4), the spacecraft approached to within 435 miles (700 km) of the
comet to take data with its three instruments two telescopes with
digital color cameras and an infrared spectrometer.

Researchers hope the flyby gives them a good idea of the
composition of the comet's icy nucleus. They are also eager to
compare Hartley 2 to the four other comets spacecraft have
visited, to generate a general sense of what makes comets tick.

Hartley 2, while just under a mile (1.5 km) across, is incredibly
active, spewing lots of dust and gas and coughing up poisonous
cyanide. [ Video
of Comet Hartley 2 jets.]